This invention relates to an electronic thermometer for estimating the temperature at an inner position of a live body based on temperature data taken on the body surface. More particularly, the invention relates to such an electronic thermometer using an equation of thermal conduction for making such an estimate.
When a conventional clinical thermometer such as a mercury thermometer is used to measure the temperature of a body by having it held under an arm or the tongue, the thermometer must be kept in that position until a thermal equilibrium is reached between the internal body position of interest and the surface temperature.
Japanese Patent Publication Tokko Hei 7-119656 B2 disclosed a method of using an equation for estimating the change in temperature while reaching an equilibrium and regarding such an equilibrium temperature as the body temperature.
It is desirable, however, to measure the internal body temperature of a patient directly. International Patent Publication WO-9850766 disclosed an electronic thermometer based on the method published in “Engineering of Heat Conduction” (at page 90) by Masahiro Shoji (published by Tokyo University). According to this method, temperatures are measured at two different positions and the temperature at a third position outside the region of the two positions is estimated. What is desired, however, is an electronic thermometer for measuring not a surface temperature but an inner temperature.
If the measurement cannot be taken until a thermal equilibrium is reached between the surface and inner temperatures, it takes as long as 10 minutes until the measurement can be taken. This wait time can be reduced by a method of estimating the inner temperature from the manner in which temperature changes to reach the equilibrium, but it still takes about 90 seconds. This method cannot fully take into account individual variations among patients or environmental changes.
As for the method according to International Patent Publication WO-9850766, the solution is unstable because the equation to be solved is non-linear and an accurate solution cannot be obtained without the help of a high-power computer, and a long computer time will be wasted.